This invention relates to the production of a copolyester from a diol, a dicarboxylic compound, and a chain branching agent, all three being reacted during the esterification stage of the polymerization action. Heretofore, many efforts have been directed to the particular methods for the production of copolyester resins. These copolyester resins have been used frequently in fiber production, coatings, containers and other conventional applications. The molecular weight of the copolyester resin so produced, according to measurement of intrinsic viscosity, vary according to the particular application, and the method for making the differentiated copolyester resins is adjusted accordingly.
A suitable copolyester resin for coatings was disclosed by this inventor and another in U.S. Pat. No. 4,124,570. However, the copolyester so produced had an acid number from about 10 to about 30, indicating the existence of carboxyl terminated end groups. These acid groups not only react slowly with conventional curing agents used for hydroxyl end groups, but also produce a by-product of carbon dioxide which is deleterious to the clarity of the resin. Therefore, it was necessary to produce a copolyester resin which minimizes the existence of carboxyl terminated end groups. Further, to the extent necessary the excess reactant creating the carboxyl terminated end groups should be eliminated.
The creation of branched copolyester resins has typically employed multi-functional agents to interreact with the chain of an oligomer and become incorporated therein withh at least one functional reactive site remaining. When these multi-functional compounds react at the end of an oligomer or incipient copolyester, the functionality is wasted at the end of the chain maintaining carboxyl groups in a superfluous position with respect to the use of curing agents compatible with hydroxyl end groups. U.S. Pat. Nos. 4,065,438; 4,080,316; 4,058,496; 3,975,566; 3,224,922; and Re. 30,102 all disclose the addition of a multi-functional anhydride compound at percentage concentrations exceeding that found to be necessary for incorporating the multi-functional group in the chain of the backbone rather than the end location. Further, U.S. Pat. Nos. 2,936,296; 3,182,041; 3,281,498; and 3,296,335 all teach the use of an excessive concentration of polyhydric multi-functional compounds for that necessary to place all of the functional groups within the chain of the polyester. Also, U.S. Pat. No. 3,027,279 discloses a multi-functional compound for the purpose of creating a high number of carboxyl end groups.
As described above, the end use of the copolyester resin is determinative of the intrinsic viscosity of the copolyester so produced, and the amount of chain branching necessary in the production of the polyester. U.S. Pat. Nos. 2,895,946; 3,251,809; 3,576,773; and 3,692,744 all disclose polyesters and copolyesters suitable for extrusion, fiber, or film-forming processes where the amount of chain branching is minimized, and hence the concentration of a multi-functional compound is all but eliminated.
Because previous work has not suggested the minimization of the chain branching agent for a polyester or copolyester useful as an industrial coating resin or decorative finish, it is necessary to alter the concentrations and processes from that disclosed in U.S. Pat. No. 4,124,570 to minimize the carboxyl terminated end groups and the excessive concentrations of the multi-functional compound.